Both n-type chalcogenide materials and/or p-type chalcogenide materials have photovoltaic functionality (also referred to herein photoabsorbing functionality). These materials absorb incident light and generate an electric output when incorporated into a photovoltaic device. Consequently, these chalcogenide-based photoabsorbing materials have been used as the photovoltaic absorber region in functioning photovoltaic devices.
Illustrative p-type chalcogenide materials often include sulfides, selenides, and/or tellurides of at least one or more of copper (Cu), indium (In), gallium (Ga), and/or aluminum (Al). Selenides and sulfides are more common than tellurides. Although specific chalcogenide compositions may be referred to by acronyms such as CIS, CISS, CIGS, CIGST, CIGSAT, and/or CIGSS compositions, or the like, the term “CIGS” shall hereinafter refer to all chalcogenide compositions and/or all precursors thereof unless otherwise expressly noted.
Photoabsorbers based upon CIGS compositions offer several advantages. As one advantage, these compositions have a very high cross-section for absorbing incident light. This means that GIGS-based absorber layers that are very thin can capture a very high percentage of incident light. For example, in many devices, CIGS-based absorber layers have a thickness in the range of from about 1 μm to about 2 μm. These thin layers allow devices incorporating these layers to be flexible. Thin layers use less material reducing the cost of the photovoltaic devices. This is in contrast to crystalline silicon-based absorbers. Crystalline silicon-based absorbers have a lower cross-section for light capture and generally must be much thicker to capture the same amount of incident light. Crystalline silicon-based absorbers tend to be rigid, not flexible.
Industry has invested and continues to invest considerable resources in developing this technology. Making stoichiometric, photoabsorbing compositions with industrially scalable processes, however, continues to be quite challenging. According to one proposed manufacturing technique, deposition methods are used in an initial stage to deposit and/or co-deposit element(s) in one or more layers to form precursor film(s). Chalcogen(s) conventionally are incorporated via chalcogenization into the precursor at a later processing stage. Chalcogenization often involves a thermal treatment in order to both incorporate chalcogen into the precursor and to crystallize the film to convert the film to the desired photoabsorbing layer. Because chalcogenization occurs after the precursors are at least partially formed, these processes can be referred to as “post-chalcogenization” processes.
There are many serious challenges to overcome with this approach. As one challenge, chalcogenizing a precursor tends to induce significant volume expansion of the film. This expansion can cause mechanical stresses that reduce adhesion, induce stresses, and/or other problems. It would be highly desirable to be able to reduce volume expansion when chalcogenizing precursor films.
Additionally, very large voids tend to form in large part at the bottom of the film adjacent the backside contact (e.g., a Mo layer in many instances) as a consequence of chalcogenizing the precursor. These large voids tend to cause adhesion problems between the CIGS layer and the backside contact layer. Electronic performance and service life also can be seriously compromised. These large voids also can induce mechanical stresses that lead to delamination, fractures, and the like. It remains very desirable to find a way to reduce or even eliminate total number, size, and even location of these voids in the finished CIGS film.
As another drawback, post-selenization processes tend to induce significant gallium migration in the film. Generally, substantial portions, and even substantially all, of the gallium in upper film regions tends to migrate to the bottom of the film. The top of the film may become completely Ga depleted. In the worst case, full Ga segregation can result such that the bottom of the film incorporates Cu, Ga, and Se, while the top of the film includes only Cu, In, and Se. This may have negative repercussions for film adhesion. Even more importantly, if some Ga is not located within the region of the CIGS film where much of the incident light is absorbed (e.g., in at least approximately the top 300 nm), the bandgap increase due to Ga is essentially lost. Such Ga depletion clearly compromises the electronic performance of the photovoltaic device. It remains very desirable to find a way to reduce and even eliminate gallium migration, or to incorporate Ga only into the areas of the film that yield a benefit (e.g., within the top approximately 300 nm of CMS film for bandgap enhancement, and within the back region of film for adhesion related void modification and/or minority carrier mirror benefits.)
Some attempts have been made to prepare precursor films that incorporate stoichiometric amounts of chalcogen. For example, the literature describes attempts to form CIGS films using compound targets of copper selenides and/or indium gallium selenides. These efforts have not been clearly demonstrated to be brought to practice for an industrial process due to the difficulty in fabricating such sputtering targets (especially for the indium selenide based materials) and the difficulty in obtaining good process control from such targets due to low sputter rates and target degradation.
As another alternative, high-quality CIGS material has been formed by thermal evaporation of all desired elements, including metals, chalcogen(s), and optionally other(s) onto a substrate at a high substrate temperature such that the film reacts and crystallizes fully during growth. Unfortunately, this evaporation approach is not truly scalable with regard to industrial applications, particularly in roll-to-roll processes.
In contrast to thermal evaporation techniques, sputtering techniques offer the potential to be a more scalable method better suited for industrial application of CIGS materials. However, it is very challenging to supply the needed amount of chalcogen during the sputtering of Cu, In, Ga, and/or Al to form high quality CIGS in one step. As a consequence, CIGS films have been sputtered from one or more metal targets in the presence of selenium and/or sulfur containing gas or vapor from an evaporated source. Using only a gas or vapor as a chalcogen source during sputtering of other components typically requires that enough gas be used to at least supply the desired chalcogen content in the precursor film, often with an additional overpressure of chalcogen. Using so much chalcogen-containing gas or vapor tends to cause equipment degradation and Se buildup, target poisoning, instabilities in process control (resulting in composition and rate hysteresis), the loss of In from the deposited film due to volatile InxSey compounds, lowered overall deposition rates, and the damage of targets due to electrical arcing.